1. Field of the Invention
This invention relates to a process for the production of styrene by the dehydrogenation of ethylbenzene in the presence of steam, and more particularly, to a procedure therein for achieving low temperature heat recovery of heat of condensation normally lost during separation of the various dehydrogenation products, most notably of ethylbenzene from styrene, and using such heat to vaporize the liquid ethylbenzene and water feeds to the dehydrogenation reactor without the need or use of a compressor.
2. Description of the Prior Act
It is well known that styrene can be produced from ethylbenzene at temperatures between about 1100 to 1200 degrees F. by vapor phase catalytic dehydrogenation in the presence of steam. Early patents on the subject have concerned themselves essentially with the nature of the catalyst. For example, among the catalysts suggested for this reaction are: palladium oxide catalysts (U.S. Pat. No. 3,502,736); platinum metal catalysts (Japanese Patent Publication No. 8367/67); supported palladium catalysts (Japanese Patent laid-open No. 133236/76); molybdenum-bismuth oxide catalysts (Japanese Patent laid-open No. 52139/76); catalysts containing oxides of at least one metal of copper, zinc, arsenic, antimony, chromium, iron, and/or cobalt (Japanese Patent Publication No. 9168/70); and catalysts containing tin, antimony, and oxygen as essential constituents (British Pat. No. 1,595,008).
More recently, however, the emphasis has shifted from catalyst to means for achieving heat economy in the dehydrogenation process, particularly with regard to the large quantities of diluent steam employed to supply the sensible heat needed for the endothermic reaction in such process and with regard to the separation of styrene from the dehydrogenation reaction effluent products. For example, British Pat. No. 1,122,857 discloses that heat may be transferred from the reactor effluent by using it to generate steam which is thereafter compressed and introduced into the reboiler of the ethylbenzene distillation zone, where styrene is removed by fractionation from ethylbenzene. In this patent, moreover, high pressure steam is used to drive the compressor for the reactor effluent steam generated.
German Offenlegungsschrift No. (OLS) 3,147,323 also focuses on achieving heat economy in the dehydrogenation process. It does so, however, by concentrating on making the dehydrogenation reactor self-sufficient in regard to its steam requirement by recovering the heat, previously abstracted by cooling, for steam generation. Its novelty consists of vaporizing water with a reaction mixture that has been cooled to 90-120 degrees C. and is at a pressure of 0.4-1.2 atm., and compressing the resultant steam to 1.4-2.5 atm. and using it to prepare the ethylbenzene, water, and steam feed mixture. U.S. Pat. No. 3,515,767 embodies the same inventive concept as this Offenlegungsschrift. It teaches the generation of subatmospheric steam from the heat of quenched reaction zone effluent, and compresses the resulting low pressure steam for use, e.g., as reboiler heat in the product recovery fractionation facilities. However, the improvements of these latter two patent publications require, for their inventive success, a compressor, a heavy duty item of equipment involving substantial investment and expense. The present invention has evolved from the need to avoid these problems and to reduce substantially the energy input and plant investment costs associated with the production of styrene through dehydrogenation of ethylbenzene in the presence of steam at elevated temperatures.
To enable use of the vaporized ethylbenzene-steam mixture in the dehydrogenation process without the need of a compressor, the present invention makes use of novel process improvements. For example, the ethylbenzene-water side of the condenser on the fractionating column separating ethylbenzene and styrene is operated at a pressure adequate for flow through the downstream system. Also, by modifying the ethylbenzene-steam feed system through the mixing of liquid ethylbenzene and water feeds and vaporizing same for introduction, at about atmospheric pressure, into the dehydrogenation reactor, it has been found that the need for steam to vaporize the liquid ethylbenzene feed is eliminated and that much of the diluent steam needed as sensible heat for the dehydrogenation reaction can be generated from liquid water. Other process improvements and simplifications are achieved as well.